This research grant proposes to continue the study of the physiology and pharmacology of synapses mediating the center-surround receptive field organization of retinal bipolar cells. Although the broad outline of synaptic interactions and the putative neurotransmitters in the retina are known, detailed cellular and molecular mechanisms underlying signal transfer in bipolar cell synapses are fragmentary, partially because of the technical difficulties associated with stable intracellular recordings from bipolar cells, and partially because of the complexity of the synaptic transfer characteristics and postsynaptic receptors in bipolar cells. This research plan proposes to overcome these difficulties by using the living retinal slices which provide the advantages of both the whole retina and the isolated cell preparations: retinal slices are intact enough so that most synapses are functional and excitable by presynaptic current or by light; and cells in slices are accessible enough so that multi-electrode recordings can be performed under visual control. Moreover, this research will also take advantage of the newly available specific glutamate and GABA receptor analogues to characterize the postsynaptic receptor subtypes mediating the light responses of bipolar cells. Synapses mediating the central and lateral inputs of bipolar cells will be individually studied when they are activated by light, presynaptic current injection or by application of neurotransmitter analogues. Postsynaptic conductance changes, kinetics and ionic components of postsynaptic currents, input-output relations and voltage gains of the photoreceptor-second-order cell, horizontal cell-cone and horizontal cell-bipolar cell synapses will be determined. Postsynaptic actions of glutamate (putative photoreceptor transmitter) and GABA (putative HC transmitter) and their analogues on bipolar cells, HCs and cones will be examined. Experiments in this proposal are designed to address a set of specific questions which include: (1) What are the postsynaptic actions and the input-output relations of the rod and cone output synapses in bipolar cells and horizontal cells? (2) What glutamate and GABA receptor subtypes are present and what function do they serve in bipolar cells and horizontal cells? (3) Is the bipolar cell surround response mediated by the feedback or the forward synapse? The long term goal of this project is to integrate results obtained into a complete and detailed description of how individual synapses are responsible for organizing the center-surround receptive fields of bipolar cells - an ubiquitous functional unit across species and the fundamental code for spatial resolution in the visual system.